Biologists have found no sign of the invasive Norway rats that have decimated native bird populations for more than 200 years on Alaska’s remote Rat Island, the U.S. Fish & Wildlife Service reports.
The scientists came to this conclusion after searching intensively for rats for more than two weeks, FWS said in a statement.
Rat Island, an island in the Aleutian chain that is part of the Alaska Maritime National Wildlife Refuge, was treated with a rat poison dropped by helicopter last year in an effort to eliminate the alien rodents and restore seabird populations and other parts of the native ecosystem.
The island is thought to have been invaded by Norway rats after a Japanese ship ran aground on the island in 1870, causing rats on board to jump ship.
The poisoning of the rats a few months ago seems to have worked.
While looking for the rodents after the extermination attempt, the biologists noted several bird species, including Aleutian cackling geese, ptarmigan, peregrine falcons, and black oystercatchers are nesting on the ten-square-mile island.
Photo of Norway rat courtesy NSF
However, the search also found “a higher-than-expected number of carcasses of two non-target species,” FWS said. Biologists found 157 juvenile and 29 adult glaucous-winged gull carcasses and a total of 41 bald eagle carcasses that appear to have died in recent months. Seventy-five percent of the eagle carcasses appear to be juvenile birds.
Map of Rat island courtesy FWS
“The cause of death of these birds is currently unknown. Many of the carcasses were in advanced stages of decomposition, but some were relatively fresh,” FWS said.
“Several of the gull carcasses found initially are now at the National Wildlife Health Center’s laboratory in Madison, Wisconsin, and it is estimated that information on the cause of death will be available by late June.”
Eagle carcasses and tissue samples were picked up from Rat Island by the refuge ship Tiglax on June 10 and were to be shipped to the Wildlife Health Center lab after the ship made port late last week.
Bird die-offs “are cause for concern and further investigation.”
While some level of winter die-off of these species is not unusual on islands in the Aleutians, and avian die-offs are not uncommon in Alaska, these numbers are cause for concern and further investigation, FWS said. “The Service is very concerned by these levels of mortality and is doing everything possible to expeditiously determine the cause of death.”
Field personnel are collecting additional tissue samples for study before destroying any remaining bird carcasses to eliminate any possibility of ongoing risk.
“Reports from the camp indicate that all bird species on the island except eagles are present in equal or greater numbers than were counted during pre-treatment surveys. Although adult and juvenile eagles are still present on the island, numbers of sub-adult eagles are lower than pre-treatment totals.”
There is no evidence of any ongoing mortality at this time. Results of the testing being performed by the National Wildlife Health Center laboratory will be released as soon as they are available
NGS Photo by Chris Johns
“While the Service regards any unnecessary loss of wildlife as a matter of utmost importance, these mortalities will not significantly impact either the Aleutian or the Alaskan bald eagle populations,” FWS predicted. “The former is estimated at 2,500 birds and the latter at approximately 50,000 eagles, and both are considered to be healthy populations.”
The Rat Island Restoration Project, a partnership among the U.S. Fish and Wildlife Service, the Nature Conservancy, and Island Conservation, began operations in 2008 after a two-year planning process.
It included an environmental analysis by federal regulators, who issued a Finding of No Significant Impact on April 15, 2008. Components of the Rat Island Restoration Project were reviewed and issued the necessary permits by the Alaska Department of Environmental Conservation, the U.S. Fish and Wildlife Service, the Environmental Protection Agency, and National Marine Fisheries Service.
“Introduced and non-native Norway rats are the most significant threat to seabird populations in the Aleutians. Rat spills can be far worse than oil spills. Oil degrades over time while rats multiply and continue to prey on native ground nesting birds that have no other land-based predators,” FWS said.
USFWS photo of Rat Island by Art Sowls